AC to DC converters are widely employed in high-voltage direct current (HVDC) transmission schemes, variable frequency drives, and other energy conversion devices. The converters convert AC power into DC power. This conversion process leads to the distortion of the waveform of the current entering a converter, which can also be regarded as that the converter generates harmonic currents.
In order to reduce the harmonic currents produced by the converter systems, various passive harmonic filters have been disclosed. For example, U.S. Pat. No. 4,622,474 revealed a high pass filter scheme that can sink high order characteristic harmonics such as the 11th, 13th. The filter package disclosed in U.S. Pat. No. 3,038,134 combines the double-tuned filter and high-pass filter to filter the 5th, 7th and higher order harmonics. The most common filtering scheme consists of a set of filter braches grouped as a filter package. Each filter branch provides a low impedance path for a harmonic current, which can also be called as that the filter branch is tuned to the harmonic. For example, filters tuned to the 5th, 7th, and 11th harmonics are used to form a filter package typical a 6-pulse converter system.
The basic configuration of a common converter is a six-pulse bridge circuit as shown in FIG. 1 141. This circuit produces 5th, 7th, 11th and 13th . . . harmonic currents even under ideal conditions, i.e. in theory. As such, the harmonics are called characteristic harmonics. By connecting multiple six-pulse blocks in parallel, multipulse converter systems can be constructed (Derek A. Paice, Power Electronic Converter Harmonics—Multipulse Methods for Clean Power, Wiley-IEEE Press, 1995, pp. 147-165.). For example, a 12-pulse converter can be formed using two six-pulse bridge circuits. The main advantage of a multipulse configuration is the reduced generation of harmonics, thereby reducing the requirements on harmonic filters. Under ideal operating conditions, the harmonic currents produced by a p-pulse converter system only contains kp±1 order harmonics, where k=1, 2, 3 . . . . For example, a 12-pulse system in theory only produces 11th, 13th, 23rd, 25th . . . harmonic currents and a 18-pulse system in theory only produces 17th, 19th, 35th, 37th . . . harmonic currents. These are the characteristic harmonics of a p-pulse converter system. All other order harmonics such as the 3rd, 5th, 7th, 9th harmonics for a 12-pulse system, or 3rd, 5th, 7th, 9th, 11th and 13th for a 18-pulse system, are denoted as the non-characteristic harmonics. Non-characteristic harmonics are generated under non-ideal operating conditions such as when the three-phase voltages supplying the converter are slightly unbalanced. But the non-characteristic harmonics are generally very small.
Since a multipulse converter produces very small amount of non-characteristic harmonics, its filter package, in theory, does not really need filter branches that are tuned to the 5th, 7th and other non-characteristic harmonics. However, the industry practice still applies filters to filter non-characteristic harmonics. This is due to the following issue. A passive filter branch is capacitive below its tuning frequency. The capacitance interacts with the supply system impedance (which is normally inductive), leading to resonance at a frequency below the filter's tuning frequency. If the resonance frequency coincides with one of the harmonic frequencies such as the 7th harmonic frequency, significant amplification of the non-characteristic 7th harmonic can occur.
The resonance issue has made the filtering schemes for multipulse systems a lot more complex and costly. For example, a 12-pulse HVDC terminal or variable frequency drive generates only a small amount of non-characteristic 5th and 7th harmonics. The primary objective of its filter package should be the reduction of the 11th, 13th and higher order characteristic harmonics. However, an 11th filter branch can create a resonance around the 7th harmonic. As a result, a small non-characteristic 7th filter is needed, which in turn requires the installation of a 5th non-characteristic filter. The main advantage of a 12-pulse system, reduced 5th and 7th harmonic generation, is therefore not fully utilized.
In addition to increasing the component costs of the filter package, the non-characteristic filters also increase the space requirement. This can be a significant issue for variable frequency drive applications. The reliability of the system is also reduced since a malfunction of one non-characteristic filter can lead to the shutdown of the entire system through an interlock mechanism (US 2012/0182089 A1). Moreover, there are still resonance points between the filter tuning frequencies. If the converter system generates interharmonics which are “harmonic” components whose frequencies are not an integer multiples of the fundamental frequency, the resonance points can easily amplify the interharmonics, leading to problems such as voltage flicker or interference with power line carrier systems. In fact, how to mitigate interharmonic amplification for such a filter package is still an open question.
In order to overcome the abovementioned problems, U.S. Pat. No. 3,935,551 and U.S. Pat. No. 3,555,291 present methods to eliminate resonance by adding one single damped-type non-characteristic filter tuning below the lowest non-characteristic harmonic frequency (for example: below the 5th harmonic frequency for a 12-pulse system) instead of a plurality of filter branches. U.S. Pat. No. 4,743,873 and U.S. Pat. No. 4,864,484 proposed filter configurations with damping resistor to limit the resonance peaks. However, the resonance issue has not been solved satisfactorily by these works.